In the microelectronics industry as well as in other industries involving construction of microscopic structures (e.g., micromachines, magnetoresistive heads, etc.), there is a continued desire to reduce the size of structural features. In the microelectronics industry, the desire is to reduce the size of microelectronic devices and/or to provide greater amount of circuitry for a given chip size.
Effective lithographic techniques are essential to achieving reduction of feature sizes. Lithography impacts the manufacture of microscopic structures not only in terms of directly imaging patterns on the desired substrate, but also in terms of making masks typically used in such imaging. Typical lithographic processes involve formation of a patterned resist layer by patternwise exposing the photoresist to an imaging radiation. The image is subsequently developed by contacting the exposed resist layer with a material (typically an aqueous alkaline developer) to selectively remove portions of the resist layer to reveal the desired pattern. The pattern is subsequently transferred to an underlying material by etching the material in openings of the patterned resist layer. After the transfer is complete, the remaining resist layer is then removed.
The resolution capability of lithographic processes is generally a function of the wavelength of imaging radiation, the quality of the optics in the exposure tool, and the thickness of the imaging layer. As the thickness of the imaging resist layer increases, the resolution capability decreases. Thinning of a conventional single layer resist to improve resolution generally compromises the etch resistance of the resist which is needed to transfer the desired image to the underlying material layer.
Furthermore, as the feature size of semiconductor devices decreases, critical dimension (CD) control becomes an important task. The “swing effect” (line width variation due to wafer surface topography and resist thickness variation) needs to be minimized during the lithographic process.
In order to obtain the resolution enhancement benefit of thinner imaging layers and reduce the swing effect in resists, multilayer lithographic processes (e.g., so-called bilayer and trilayer processes) have been developed. In bilayer lithographic processes, a so-called planarizing underlayer is used intermediately between the photoresist layer and the underlying material layer to be patterned. The underlayer receives the pattern from the patterned photoresist layer (typically a silicon-containing resist), and then the patterned underlayer acts as a mask for the etching process needed to transfer the pattern to the underlying material. In trilayer lithographic processes, a so-called interlayer (typically a silicon-containing composition) is used intermediately between the photoresist layer and the underlayer. The interlayer receives the pattern from the patterned photoresist layer, and then the patterned interlayer acts as a mask for etching the underlayer. The patterned underlayer then acts as a mask to transfer the pattern to the underlying material through etching, electroplating, metal deposition, ion implantation, or other semiconductor processing techniques.
The planarizing underlayer compositions should be sufficiently etchable selective to the overlying photoresist (to yield a good profile in the etched underlayer) while being resistant to the etch process needed to pattern the underlying material layer. Further, the planarizing underlayer composition should have the desired optical characteristics such as real index of refraction (n) which is the real part of refractive index, extinction coefficient (k) which is the imaginary part of refractive index, reflectivity, optical density, and etc., such that the need for any additional anti-reflective coating (ARC) layer is avoided. The planarizing underlayer composition should also have physical/chemical compatibility with the photoresist layer to avoid unwanted interactions which may cause footing and/or scumming. The typical thickness of the planarizing underlayer is very thin in order to fit the first minimum or second minimum in the reflective curve. Recently, many applications require a thick planarizing underlayer for etching. In this situation, the required k value of the ARC has to be reduced to a range similar to the traditional underlayer used in the bilayer resist system. The challenge of designing these types of underlayers for bilayer, single layer, and trilayer applications is to have desirable etch resistance towards an oxygen or nitrogen/hydrogen plasma, while having the k value in the range from about 0.12 to about 0.42.
Known underlayers for I-line and 248 nm DUV multilayer lithographic applications are typically based on novolac or polyhydroxystyrene polymers. However, these materials strongly absorb 193 nm lithographic applications. Thus, there is a continued need for improved compositions, especially compositions useful in lithographic processes using imaging radiation less than 200 nm (e.g., 193 nm) in wavelength.
U.S. Pat. No. 6,818,381 (hereinafter “the '381 patent”) discloses planarizing underlayer precursor compositions comprising a polymer containing aromatic moieties such as phenyl and phenol groups as absorbing groups. Polymers containing aromatic moieties disclosed in the '381 patent provide absorbance to 193 nm wavelength. However, since the aromatic moieties such as phenyl and phenol have high absorption, only small amounts can be incorporated in the polymer structure, thus rendering the polymers less etch resistant and less likely to produce consistency. Further, the commonly used aromatic moieties in the polymer disclosed therein such as polystyrene and polyvinylphenol have k values too high to meet the requirement for the underlayer application. The preferred k value for underlayer is usually in the range from about 0.12 to about 0.42.
Thus, there remains a need for underlayer compositions that are compatible with typical photoresists and have desired optical properties so that the underlayer can also be used as an ARC.